The solar cell using the amorphous silicon film is known. This solar cell using the amorphous silicon film can be thinly produced in a high productivity and light weight. Thus, such solar cell has been used as a power source of portable devices such as a table calculator, watch or the like.
Particularly, once the solar cell is mounted on a watch, the watch has a merit of no cell or battery exchange, with the result that the solar cell can largely contribute to the spread of a quartz type wrist watch.
However, when a usually shaped solar cell is mounted on a watch, the problems occur that a design of the watch is limited and that not only appearance of the watch becomes bad, but also the whole size thereof is increased. Accordingly, incorporation of the solar cell with a shape matching the dial face in a wrist watch has been tried (Nikkei Business, Mar. 18, 1996, pages 71-73).
Such a shaped solar cell has, however, the problems that the production cost thereof is further increased over a conventional solar cell and its reliability is reduced.
Alternatively, in order to enhance the productivity of the solar cell there is a technology of forming the electrode by printing.
FIG. 1a is a cross-sectional view illustrating a part of a structure of a solar cell having electrodes formed by a printing process. In this case, the solar cell shown in FIG. 1a is a type of solar cell that incident rays enter the surface (the surface of the device) side where a photoelectric conversion layer is formed.
In FIG. 1a, the reference numeral 100 denotes a glass substrate, stainless steel substrate or resin substrate, 101 a first electrode formed of, for example, aluminum, 102 a photoelectric conversion layer of amorphous silicon layers deposited in order of P-I-N types or N-I-P types from the glass substrate 100, 103 a second electrode of indium tin oxide (ITO) that is an electrode on a light incidence surface side, 104 a drawing electrode formed by using a printing process.
In the printing process, a conductive paste in which a conductive material generally composed of spherical, cigar shaped or rugby ball shaped metal particles is dispersed is printed in a desired pattern.
Thus, in a case where a printing process in which a pattern can directly be formed is utilized, the merit that production steps of electrodes can be simplified is obtained. However, there occurs the problem that pressure in a certain degree is locally applied to a portion to be printed. This pressure is particularly concentrated at a portion in which dispersed conductive materials are present.
An ITO film (second film) 103 having a structure shown in FIG. 1a, is not so hard and strong, and has a thickness of only a few thousand angstroms or less. Further, the surface of the ITO film has significant concave and convex portions. And, amorphous silicon which forms the photoelectric conversion layer 102 also is not so hard.
Therefore, high pressure is locally applied to the photoelectric conversion layer 102 due to the pressure concentration during printing. As a result, damage, such as a crack or the like can occur in the photoelectric conversion layer 102, whereby a conductive material in a conductive paste can penetrate into the crack or the like. FIG. 1b shows the state where a conductive paste penetrates into the crack or the like. The reference numeral 105 denotes a thus formed short circuit. When such short circuit 105 is formed, the first electrode 101 is electrically connected to the second electrode 103, whereby the photoelectric conversion layer does not fully function as a solar cell.
As described above, the simplification of the production steps can be accomplished by producing electrodes by using the printing process. However, the printing process damages the photoelectric conversion layer, and is likely to form a short circuit between the first electrode and the second electrode.
Therefore, the object of the present invention is to provide a solar cell for a wrist watch which can be produced at a low cost and has a high reliability, and the production method thereof.
Another object of the present invention is to provide a solar cell whose production steps can be simplified and which has high production yield.